President Barack Obama on Monday conceded difficulties in fighting "opportunistic" and "nimble" Islamic State militants, but defended progress that had been made in combating the extremists in Iraq and Syria.

During a rare visit to the Pentagon, Obama also warned of the Islamic State's efforts to recruit and inspire vulnerable people in the United States.

"The threat of lone wolves or small cells of terrorists is complex," Obama said following a meeting with top defense officials.

Obama's Pentagon visit followed a wave of weekend airstrikes by the U.S.-led coalition in eastern Syria. The coalition said it was one of the most sustained aerial operations carried out in Syria to date.

The president said he currently has no plans to send more U.S. troops to Iraq. While he has vowed to keep American service members out of direct combat, he has sent more than 3,000 U.S. troops to advise and assist the beleaguered Iraqi military.

After announcing the deployment of 450 additional service members last month, Obama said Monday that there were "no current plans" to send more troops.

Efforts to train local forces in Iraq and Syria, however, have been slow to take shape. Last month, Obama acknowledged that the U.S. lacks a "complete strategy" for training Iraqi troops to carry out ground missions. And in Syria, fewer than 100 rebels are being trained by the U.S., far fewer than the goal of producing 5,400 fighters a year.